Scientists have discovered water vapor on Ceres, a dwarf planet and the largest object in the asteroid belt.

Using the European Space Agency’s Herschel space telescope, researchers detected a spectral signature of the substance on four occasions.

“This is the first time water vapor has been unequivocally detected on Ceres or any other object in the asteroid belt and provides proof that Ceres has an icy surface and an atmosphere,” Michael Kueppers, a planetary scientist at ESA and the lead author of a paper documenting the discovery, said.

Ceres is an ice-covered object. One way the water vapor could have formed is through the melting of that ice as Ceres moves closer to the sun during its orbit.

“Think of it as ice that has been covered by dust,” Joel W. Parker, a planetary scientist at Southwest Research Institute in Boulder, Colo. who has studied Ceres, said. “That ice heats up as Ceres slightly gets closer to the sun. The ice sublimates, goes to gas, lifts off some of the dust, which perhaps exposes a little more ice.”

Parker said that the other possible mechanism for melting Ceres’ ice could be cryo-volcanism.

“Ceres is layered, kind of like the Earth,” he said. “Ceres has ice. If there is some deeper ice, there could be cryo-volcanism.”

Cryovolcanos occur on frozen celestial objects and involve the eruption of volatile compounds, including water, in plumes.

“You need to heat that mantle somehow to make the water escape, to make the low volcanoes,” Parker explained. “You can’t do that with heat from the sun. That model would require internal heating, like some radioactive elements.”

Kueppers and his fellow researchers concluded that the ice melt scenario is more likely correct.

It will not be necessary to wait long for a conclusive determination about whether their conclusion is the right one. NASA’s Dawn probe is in the asteroid belt and is scheduled to arrive at Ceres next spring.

The spacecraft has on board both a visible and infrared mapping spectrometer and a gamma ray and neutron detector.

“We first hope to see surface markings that will tell us where the vents, if any, are,” Christopher Russell, the DAWN mission’s principal investigator and the director of the Institute of Geophysics and Planetary Physics at the University of California at Los Angeles, said. “They should be able to tell whether there are water-bearing minerals with the mapping spectrometer. The neutron detector will tell us where there is hydrated soil.”

Scientists have long been confident that Ceres is shrouded in ice. Spectroscopic observations of the dwarf planet have indicated that it has a silicate core. Since the total density of Ceres and the assumed density of the silicate core can be calculated, Ceres’ mantle must be composed of several tens of kilometers of ice.

“We always expected Ceres to be a wet body because of its low density,” Russell said.

Kueppers and his colleagues detected a huge amount of water vapor sublimating from Ceres. About one octillion molecules of the compound are moving into space each second.

They appear to emanate from the dwarf planet’s mid-latitudes.

Ceres was discovered by the Italian astronomer Giuseppe Piazzi on Jan. 1, 1801. It was initially thought to be a planet with an orbit that lay between Mars and Jupiter, but soon after its discovery was deemed an asteroid. It was reclassified as a dwarf planet in 2006 because of its 950 kilometer diameter, large for an asteroid belt object.

The paper appears in the Jan. 23 edition of Nature.

This image of Ceres was obtained by the Hubble Space Telescope on Jan. 1, 2004. Image courtesy NASA, European Space Agency, Southwest Research Institute, Cornell University, University of Maryland, and Space Telescope Science Institute.

Mercury, long thought to be a planet without the ingredients necessary for volcanism, was volcanically active as recently as one billion years ago.

Scientists learned, as a result of a 2008 discovery by the MESSENGER spacecraft of pyroclastic ash deposits on the planet’s surface, that the solar system’s smallest planet has experienced volcanism.

The general assumption has been that Mercury’s volcanic activity must have occurred early in the planet’s history, about 4 billion years ago.

Newly-published research indicates that some of the pyroclastic ash deposits seen by MESSENGER were actually deposited between about 1 and 3.5 billion years ago.

That means volatile compounds, which drive the explosive eruption of volcanoes, are likely more prevalent on Mercury than has been thought.

“Mercury, contrary to predictions, is not deficient in volatiles, but instead has an abundance of them, “ Sean C. Solomon, the director of the Lamont-Doherty Earth Observatory at Columbia University and MESSENGER’s principal investigator, said.

Using data obtained by cameras and spectrometers on board MESSENGER, a team of scientists examined 51 sites at which the ash was deposited. They observed that the vents through which the ash was expelled onto Mercury’s surface exhibited varying degrees of erosion and determined the age of the craters in which the ash exists.

The difference in the erosion of the vents shows that ash was deposited on the surface of the planet at a variety of times during its history.

“If [the explosions] happened over a brief period and then stopped, you’d expect all the vents to be degraded by approximately the same amount,” Timothy Goudge, a geology graduate student at Brown University and the lead author of the paper, said. “We don’t see that; we see different degradation states. So the eruptions appear to have been taking place over an appreciable period of Mercury’s history.”

The fact that some of the craters are older than the ash within them indicates that the ash must have been deposited in those craters after the impacts that produced the craters.

“These ages tell us that Mercury didn’t de-gas all of its volatiles very early,” Goudge said. “It kept some of its volatiles around to more recent geological times.”

Mercury’s large iron core has led to speculation that the planet may once have been significantly larger than it is now.

Solomon explained that scientists have tended to believe either that some of its mass was either burned away by the heat of the nearby Sun or blasted away during a collision with another celestial object. If either of those events had occurred, most, if not all, of Mercury’s volatile compounds would have been eliminated early in its history.

The new study’s confirmation that volatile compounds existed on Mercury until as recently as about 1 billion years ago casts doubt on those leading hypotheses about the planet’s formation.

“It really does tie the geochemistry and the geology together, pointing toward an origin we didn’t expect,” Solomon said.

Pyroclastic ash is produced by a volcano when it erupts. Volatile compounds in magma, such as carbon dioxide, hydrogen sulfide, sulfur dioxide, and water, change state from liquid to gas as the temperature of the magma rises and then expand, causing an increase in pressure that leads to the eruption.

The new paper appears in the March 28 edition of Journal of Geophysical Research: Planets.

MESSENGER was launched Aug. 3, 2004. Having completed its third year in orbit around Mercury last month, the probe is scheduled to crash into Mercury’s surface in March 2015.

The European Space Agency and the Japan Aerospace Exploration Agency plan a joint mission to Mercury to be launched in 2016.

Enceladus, the icy moon of Saturn known for its spectacular geysers, likely has an ocean of liquid water beneath its surface.

Researchers relied on gravity and topography data obtained by NASA’s Cassini probe in concluding that Enceladus could be the third known outer solar system moon that features liquid water.

The ocean is a regional water body near the moon’s south pole. It seems to lie about 30 to 40 kilometers below the moon’s surface, covered by a thick ice sheet, and likely has a depth of about 10 kilometers.

“This means it is as large as or larger than Lake Superior,” David J. Stevenson, a professor of planetary science at the California Institute of Technology and a co-author of the paper documenting the discovery, said.

Scientists depended on measurements of the gravity field of Enceladus to infer the presence of the ocean. As Cassini flew 100 or fewer kilometers over the moon’s surface during three flyovers, the spacecraft’s speed changed by a few millimeters per second.

The tiny slowing of Cassini was detected by antennae on Earth as the radio waves transmitted by the probe during the flyovers shifted slightly, just as sound waves do as their source moves farther away from a receiver.

“The detection of such small velocity variations gave us information about the accelerations the spacecraft was subject to,” Marzia Parisi, an aerospace engineer and research fellow at Sapienza University of Rome and one of the authors of the paper, said.

An acceleration of an object can include both an increase and a decrease in its speed.

The acceleration of a spacecraft that is flying over a celestial object is affected by a variety of factors, including changes in the object’s gravity.

“This could arise because of the absence of mass that is a depression of the ice surface, but when you look at the actual surface, you can see that although there is indeed a depression, it is much larger than that needed to explain the gravity,” Stevenson said. “So that means that there must be a compensating positive mass excess under the south polar ice. The natural way to do this is to have a layer of water because water is more dense than ice.”

In the case of the Cassini flyovers, another likely impact on the spacecraft’s velocity was drag caused by passing through water vapor plumes that are emitted from the moon’s south polar region.

The impact of those factors had to be considered before the research team could conclude that the cause of Cassini’s acceleration change was a fluctuation in mass tied to the presence of liquid water beneath the moon’s surface.

Parisi explained that the researchers were able to take account of those factors in their modeling.

The data does not explain how an ocean could exist on Enceladus, but it is logical to assume that Saturn’s gravitational pull has a great deal to do with it.

One significant implication of the discovery of an ocean on Enceladus is the possibility that it could be hospitable for microbial life.

“The added value of our work is that a large, potentially habitable environment has been found in an unexpected place in the solar system, where the energy needed to produce liquid water from ice is not provided by solar radiation,” Luciano Iess, a professor of aerospace engineering at Sapienza University of Rome and the lead author of the paper, said.

It is likely to be many years, if not decades, before humanity can find out whether, in fact, microbes exist beneath Enceladus’ icy surface.

Cassini will continue to explore the Saturnian system for about three more years. The probe will fly over Enceladus three more times, but will not further investigate Enceladus’ gravitational field to gain further insights into the ocean beneath its surface.

Europa, a Jovian moon, and Titan also have oceans, though on Titan it is composed of liquid methane.

NASA’s planetary science budget includes funds to plan a trip to Europa.

The paper documenting the discovery of an ocean on Enceladus will be published in the April 4, 2014 edition of Science.

Image courtesy Wikimedia.

Graphic courtesy NASA, Jet Propulsion Laboratory-California Institute of Technology. Note the illustration of the hypothesized sub-surface ocean, appearing as a blue region just above the water vapor jets.